M2Dude, ChristiaanJ & Brit312. Thankyou for your informed explanations much appreciated. It took a little while trawling through pics on Google images to locate this intruiging tab. It does not appear on most pics, was it a late modification?
Mrs Philflies despairs of the energy wasted looking for this obscure feature! oh well.
Anyway, with the risk of looking daft, is it not by the No 1 EGT? (M2..said No 4 which is maybe why it took me so long to find)
Regards